Many different types of facilities produce or use streams
containing a high carbon dioxide (CO2) content
(98+%) with low hydrogen sulfide (H2S)
concentrations, e.g., a few parts per million by volume (ppmv)
to a few volume percent (vol%). Examples include
CO2-flood enhanced oil recovery, pre-combustion carbon capture (from fossil
fuel-fired power plants and industrial facilities) and sequestration,
natural gas conditioning, and agricultural manufacturing, among
others. In all of these industries, the potential for a release
in a processing step or during transmission through a pipeline
exists.

The health effects and dangers of H2S are well
known, but those of CO2 are not as commonly
understood. It is uncertain if industry realizes that
CO2 is a mildly toxic gas and not just a simple
asphyxiant like nitrogen. Because CO2 itself is
toxic at higher concentrations, the high-purity CO2
streams can actually be more hazardous than the H2S
and they are the subject of discussion in this article. In such
cases, the presence of H2S may actually allow easier
detection of the CO2 danger.

This article reviews the hazards of H2S and
CO2 and compares the effects from these acid gases
on humans. Concentration levels corresponding to the
immediately dangerous to life and health (IDLH) levels of the
two gases are used to illustrate conditions where both
H2S and CO2 are present, and the
CO2 (not the H2S) is the predominant
concern. A goal is to educate readers to think of
CO2 as a mildly toxic gas and not just an
asphyxiant, and to recognize conditions where it can represent
the more significant hazard, even if small concentrations of
H2S are also present.

Toxicity of H2S.

Hydrogen sulfide is an intensely hazardous, toxic
compound.1 It is a colorless, flammable gas that can
be identified in relatively low concentrations by a
characteristic rotten egg odor. This acid gas is naturally
occurring and is in the gases from volcanoes, sulfur springs,
undersea vents, swamps and stagnant bodies of water and in
crude petroleum and natural gas. Hydrogen sulfide is produced
when bacteria break down sulfur-containing proteins, and it is
a component of decomposing materials. In addition,
H2S is also produced from man-made operations and
processes such as petroleum refineries, food processing plants,
tanneries, municipal sewers, sewage treatment plants,
landfills, swine containment and manure-handling operations,
and pulp and paper mills.

Hydrogen sulfide has a very low odor threshold, with its
smell being easily detected by most people in the range of
0.0005 ppmv to 0.3 ppmv.2 As the gas becomes more
concentrated, the odor increases with a strong rotten egg smell
identifiable up to 30 ppmv. From about 30 ppmv to 100 ppmv, the
gas is stated to have a sickeningly sweet odor. However, at
concentrations above 100 ppmv, a persons ability to
detect the gas decreases due to a rapid temporary paralysis of
the olfactory nerves in the nose that leads to a loss of
the sense of smell. This means that the gas can be present
in the environment at extremely high
concentrations with no noticeable odor. This unusual property
of H2S makes it very dangerous to depend solely on
the sense of smell as a warning sign of the
gas.3

Once H2S is released as a gas, it remains in the
atmosphere for an average of 18 hours, after which it changes
to sulfur dioxide and sulfuric acid.2 It is
water-soluble and, therefore, it may partition to surface water
or adsorb onto moist soil, plant foliage, or other organic
material where it loses much of its toxic properties.

Hydrogen sulfide is classified as a chemical asphyxiant,
similar to carbon monoxide (CO) and cyanide
gases. It interferes with nerve cell function, putting certain
nerves to sleep, including olfactory (as discussed previously)
and the ones necessary for breathing. Table 1 shows the typical
exposure symptoms of H2S.

It is important to note that while most chemicals are toxic,
exposure has to occur (at a level that is considered toxic)
before adverse health effects are observed. Most, if not all,
of the irreversible health outcomes including death have
occurred due to overexposure to H2S in confined
areas.

Toxicity of CO2.

Carbon dioxide is a slightly toxic,
odorless and colorless gas. It is typically found in air at
around 360 ppmv (0.036 vol%) while exhaled air may contain as
much as 40,000 ppmv (4 vol%). Table 2 shows the general affects
of CO2 over different ranges of exposure.

At lower concentrations, CO2 affects the
respiratory system and central nervous system. Too much
CO2 also acts as a simple asphyxiant by reducing the
amount of oxygen available for respiration.6 At
higher concentrations, too, the ability to eliminate
CO2 decreases and it can accumulate in the body. In
this way, CO2 differs from some other asphyxiants,
such as nitrogen (N2). Unlike CO2,
N2 does not get distributed throughout the body to
cause an adverse health effect; rather, N2 acts
simply by displacing oxygen from the air and, thereby,
decreasing the amount of oxygen available for respiration.
Result: CO2 is dangerous at a much
lower level than some other asphyxiants, such as
N2.

Nitrogen is discussed here because it is a common potential
asphyxiant in industrial settings. The following example
illustrates the differences between CO2 and
N2. Consider a hypothetical example where 90 parts
of atmospheric air (normally 21% O2 and 79%
N2) are mixed with 10 parts of either pure
CO2 or N2. The resulting mixture
compositions are shown in Fig. 1.

As shown in Fig. 1, the resulting mixture with
CO2 addition contains 18.9% O2, 71.1%
N2, and 10% CO2. As discussed previously,
such a mixture could potentially kill a person. Conversely, the
mixture with N2 contains 18.9% O2 and
81.1% N2; while this mixture is lower in oxygen than
normal air and below the recommended O2 % for
workers, it is not likely to cause irreversible health effects.
The effect of going from a 21% oxygen atmosphere to an 18.9%
oxygen atmosphere is similar to going from sea level to about
3,000 ft in elevation (roughly the elevation of Midland,
Texas), as far as the oxygen partial pressure is concerned.
Most people who are acclimated to sea level would have no
trouble going to 3,000 ft in elevation.

In summary, mixing 10 parts CO2 with 90 parts air
can possibly cause a person breathing the mixture to die if
exposed long enough. In contrast, mixing 10% N2 with
air probably has little effect on a person. Clearly, it is very
important to recognize that CO2 is not the same
simple asphyxiant as N2.

Occupational exposure limits for H2S and
CO2.

Table 3 provides a summary of occupational exposure limits
for H2S and CO2. Occupational exposure
limits are typically designed to protect health and to provide
for the safety of employees for up to a 40-hour work week, over
a working lifetime. The threshold limit value (TLV) was
developed by the American Conference of Governmental Industrial
Hygienists (ACGIH) while the permissible exposure limit (PEL)
is an enforceable standard developed by the Occupational Safety
and Health Administration (OSHA). The short-term exposure limit
(STEL) was developed by ACGIH and represents a 15-minute
time-weighted average exposure that should not be exceeded at
any time during the workday. The IDLH value was developed by
the National Institute for Occupational Safety and Health
(NIOSH) to provide a level at which a worker could escape
without injury or irreversible health effects.

IDLH values are conservatively established by NIOSH to give
a worker approximately 30 minutes to evacuate an area. The IDLH
for both H2S and CO2 are purposefully
established below levels at which adverse and irreversible
health effects would be seen following 30 minutes of exposure.
The IDLH for H2S was developed based on human data
(and supplemented with information from laboratory animals)
that showed that between 170 ppmv and 300 ppmv, a person can be
exposed for one hour without serious health effects and that
400 ppmv to 700 ppmv can be dangerous if exposure is greater
than 30 minutes. A person can be exposed to H2S at
800 ppmv for approximately 5 minutes before unconsciousness
occurs, while exposure at 1,000 ppmv or greater can cause
immediate respiratory arrest, unconsciousness and possibly
death.

For CO2, a person can sustain exposure to the
IDLH of 40,000 ppmv for 30 minutes with minimal signs of
intoxication (e.g., changes in breathing rate, headache and
fatigue). At 30 minutes of exposure to 50,000 ppmv
CO2, signs of intoxication become more pronounced. A
person can sustain exposure to 70,000 ppmv to 100,000 ppmv
CO2 for about 5 minutes and signs of intoxication
become intense with very labored breathing, visual impairment,
headache, ringing in the ears and potentially impaired
judgment. Air containing CO2 at a concentration
greater than 100,000 ppmv (i.e., 10 vol%) can produce extreme
discomfort and, as indicated above, can be
life-threatening.

Table 4 shows an example of how a gas stream containing
initial concentrations of H2S of 2,000 ppmv and of
CO2 of 98 vol% would change assuming a uniform
dispersion in air for both compounds. As shown in the table,
when the IDLH of H2S (100 ppmv) is reached, the
CO2 content is still above the IDLH level of 40,000
ppmv. Even more dramatic are the 5-minute exposure levels; when
the H2S exposure level is at the 5-minute limit of
800 ppmv, the CO2 concentration is at 392,000 ppmv,
which is far above the level a person can survive for 5
minutes. Thus, given the much higher percentage of the
CO2 in this gas stream, the danger from
CO2 is higher than the danger posed by
H2S.

Potential exposure scenarios to H2S and
CO2.

In actuality, it is difficult to determine the
likelihood of a release and the potential concentration a
person may encounter following a release. A release could occur
at any point in the processing unit or transfer pipeline
depending on the source of the stream (see Fig. 2). Atmospheric
conditions, such as the wind or physical location of the
release (low lying area), can greatly affect the dispersion
rate and exposure concentrations of the two compounds. Some
potential exposure scenarios are discussed here.

If there is wind, a small release (i.e., not a catastrophic
event) would most likely disperse relatively quickly. Under
this scenario, a person downwind (unless they were within close
proximity to the release) would probably not be exposed to a
harmful concentration of either compound. In fact, the presence
of H2S (which has an odor at very low
concentrations) may actually provide an early indicator of a
CO2 release that would otherwise go undetected.
Although H2S may provide an early indicator of a
release in certain situations, this should not be relied upon
because H2S deadens the sense of smell at higher
concentrations. Exposure should be kept to a minimum by
applying sufficient engineering controls and safe work
practices. Appropriate monitoring and personal protective
equipment should always be used.

Because both compounds are heavier than air (the specific
gravity for H2S and CO2 is 1.192 and
1.52, respectively), the most likely place to encounter harmful
levels of either compound would be in a low-lying area or
depression. This is currently an issue for CO2
pipelines in which harmful levels of CO2 can
accumulate in these areas, regardless of the presence of
H2S. The presence of H2S increases
concerns due to its more insidious toxicity (i.e., it can
render a person incapable of escape at sufficiently high
concentrations). However, levels above the IDLH could occur in
a confined space or depression for either compound. As
indicated earlier, the presence of H2S may provide a
warning that a release has occurred and prevent a person from
entering the area where potentially dangerous levels of
CO2 or H2S may be present.
Note: The use of direct reading gas detection
instrumentation and other protective measures should be
required before entering confined spaces such as manholes,
tanks, pits and vessels that could contain a buildup of these
gases.

Potential synergistic effects of concurrent exposure.

Since the mechanisms of action for CO2 and
H2S are very different, it is unlikely that exposure
to both compounds will be worse than exposure to only one
compound. Most occupational exposure limits are based on
exposure to single compounds, even though it is recognized that
multiple compounds may be encountered, and Environmental Protection Agency only
considers compounds additive if they affect the same target
organ or act by the same mechanism. Moreover, industries such
as swine production, where both CO2 and
H2S are measured in the air, do not adjust
occupational exposure limits for added worker safety nor have
synergist effects (i.e., effects that are worse when in
combination than when exposure is to a single compound) been
noted for industries where exposure to both compounds
occur.8

Evaluation of risk.

Based on the general qualitative analysis of exposure to
both H2S and CO2 discussed here, it
appears that there is no increased risk from the presence of
H2S at low levels (e.g., up to perhaps 2,000 ppmv or
higher) in high-purity CO2 gas. In fact, in these
types of gas streams, the potential exposure to high
CO2 concentrations during a release event could be
as dangerous, or more dangerous, than exposure to lower
concentrations of the more toxic H2S. At high
concentrations, CO2 may accumulate in the body,
which is different than some other asphyxiants (i.e.,
N2). It is most important to recognize the
difference between CO2 and other common asphyxiants.
In some cases, the H2S in the gas may serve as a
warning for the more hazardous CO2 environment.
Dispersion modeling for specific release scenarios should be
conducted to better understand possible exposure limits and
impacts on human health for both compounds. Appropriate safety
precautions should be implemented including monitoring (both
fixed and personal detection systems) and training on chemical
hazards, personal protection equipment and safety rescue
procedures. HP

Kirby Tyndall, PhD, DABT, is a
senior consulting toxicologist with Pastor, Behling,
& Wheeler, LLC. She is a board certified toxicologist
with over 19 years of experience in the fields of
toxicology, risk assessment and risk management. Dr.
Tyndall has worked in both the environmental consulting
and government sectors, and has significant experience
evaluating potential human health and ecological risks
associated with exposure to contaminants in environmental media (air,
water, soil, sediment and biota including fish,
etc.).

Ken McIntush, PE is a practicing
chemical engineer and president of Trimeric Corp., a
small company based in Buda, Texas, that is focused on
chemical/process engineering. He has about 21 years of
varied process engineering experience, serving clients in
oil refining, oil and gas
processing, silicon refining and several other
industries. Mr. McIntush performs troubleshooting,
debottlenecking and other projects for the company. He
holds a BS degree in chemical engineering from Texas
A&M University, College Station.

Joe Lundeen is a principal engineer
at Trimeric Corp. in Buda, Texas. He has 21 years of
experience in process engineering, process
troubleshooting, and facility installation for oil and
gas production and CO2 processing clients.
His recent experience has been focused on dehydration,
contaminant removal, and transport of super-critical
CO2. He holds BS and MS degrees in chemical
engineering from the University of Missouri,
Rolla.

Kevin Fisher, PE is a principal
engineer at Trimeric Corp. in Buda, Texas. He has over
20 years of experience in process engineering, research
and development, and troubleshooting for oil and gas
production and oil refining clients, as well as
for private and government-sponsored research programs.
He holds an MS degree in chemical engineering from the
University of Texas, and BS degrees in chemical
engineering and chemistry from Texas A&M and Sam
Houston State University, respectively.

Carrie Beitler is a senior engineer
at Trimeric Corp. in Buda, Texas. She has over 15 years
of experience in process engineering, process modeling
and optimization of unit operations in the natural gas,
petroleum refining and CO2
processing areas. She also specializes in the
development of process design packages for the
fabrication of open-art technology such as caustic
scrubbers, acid-gas injection units, glycol dehydrators
and amine
treaters. She graduated with a BS degree in chemical
engineering from Purdue University.

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It is very good article. I would like to take opinion / comments of experts about the case where fuel gas contains Co2 more than one lakh ppm and H2S is less than 90 ppm, In such case, what precautions will be required for using this fuel gas in boiler and power turbine which are proposed to be used in enclosed area at FPSO? I would be grateful if expert give comments with supporting codes / class reference for handling about 0.15MMSCMD natural gas as fuel gas.

02.12.2011

Excellent assessment.The same opinion with the precedent comments on H2S. I am working as corrosion engineer. My point is that as both together are the chlorides- not enough considered from environmental and chemical agressivity point of view. I recommend studies on the last- chlorides, present also in our potable (!?) water. Thank you for your efforts. Trimeric is not a "small" Company, having so good specialists. I would like to be in contact with them.

01.26.2011

The authors have performed a valuable service by developing this well written article describing the insidious but harmful nature of high concentration CO2 releases into ambient air.

I do take exception to the comparison with H2S however. H2S is arguably the most LETHAL chemical commonly existing. A single breath of 10% H2S can be fatal with a 2nd breath not ever taken. 'I thought I smelled H2S, but it was just a little bit' is a common exclamation from survivors of single breath incapacitation--who were rescued and resuscitated. Such in the unforgiving nature of H2S --as long as you can smell it--you are going to be able to get out to fresh air--when you cannot smell it--you MAY have taken your last breath of life.